Complex relative permittivity and dielectric loss

This will produce polarisation, which alternates in direction, and, if the frequency is high enough, the orientation of any dipoles which are present will inevitably lag behind the applied field. Mathematically, we can express this as a phase lag 5 in the electric displacement  

It is convenient to combine these two quantities into a complex relative permittivity (or complex dielectric constant)  

The meaning of the real and imaginary parts may be readily appreciated by considering the material in a capacitor (capacitance C0 when empty), as shown in Fig. 3.1. The current / which flows in the external circuit after application of 

This implies that we have a capacitive component of the current, 

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